Inlet casing for axial flow compressor



Nov. 10, 1959 R. w. GENTILE INLET CASING FOR AXIAL FLOW COMPRESSOR s Shee ts-She'et 1 Filed Dec. 18, 1957 Tllllllrlll l INVENToR RICHARD\ w. GENTILE H IS ATTORNEY Nov. 10, 1959 R. w. GENTILE 2,912,156

INLET CASING FOR AXIAL FLOW COMPRESSOR I Filed Dec. 18, 1957 :5 Sheets-Sheet z FIG.5.

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I L 4 I R i P MAJOR AXIS GP ELL/PSE m k AXIS or ROTOR E g 800 i R 2 Fl G. 5. a Q 600 f a w 3400 E: \NVENTOR 3820a RICHARD w. GENTILE $5 l BY 0 225 525 LOCATION'ALONG MEAN FLav'v PATH HIS ATTORNEY R. w. GENTILE 2,912,156

INLET CASING FOR AXIAL FLOW COMPRESSOR S Sheets-Sheet 3 Nov. 10, 1959 Filed Dec. 18, 1957 llllll .IIHI".

H lllllg ll HllHlH T I I I I lk INVENTOR RICHARD 'W. GENTILE FIG.7.

HIS AttORNEY United States Patent 2,912,156 INLET CASING FoR AXIAL FLOW COMPRESSOR Richard W. Gentile,"Schenectady, N.Y., assign'or to General Electric Company, a corporation of New York Application December 18, 1957, Serial No. 703,653

9 Claims. (Cl. 230-133) This invention relates to multi-stage axial flow compressors, particularly to an improved inlet casing structure.

Experience has shown that the design of the inlet passage to an axial flow compressor has a most important bearing both on the aerodynamic performance of the compressor, and on its mechanical performance from the standpoint of fatigue failure of the stationary inlet guide vanes and the initial row of rotor blades. Any discontinuities in the flow approaching the guide vanes and first stage rotor blades has a tendency to set up vibrations in these vanes and blades, and if the periodicity of the discontinuities in the fluid flow is near the natural frequency of the blades, or some multiple thereof, the vibrations set up may be sufficiently serious to cause early failure. It goes without saying that non-uniform flow into the rotor inlet has a deleterious elfect on the aerodynamic performance of the compressor. Particularly, non-uniform or variable angles of attack of the fluid approaching the guide vanes may produce wakes leaving the trailing edge of the vane, which results in decreased aerodynamic performance and vibration-inducing stimuli.

Accordingly, an object of the present invention is to i provide an improved axial flow compressor inlet casing assembly which efiects optimum flow distribution at the compressor inlet, and with minimum local disturbances or discontinuities which might induce destructive vibrations in the compressor inlet guide vanes or rotor blades.

Another object is to provide an improved axial flow compressor inlet casing which results in no extension of the overall axial length of the machine.

Still another object is to provide an improved compressor inlet casing providing uniform angle of approach to the compressor inlet guide vanes.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawings, in which Fig. 1 is a longitudinal view, partly in section, of an axial flow compressor inlet casing incorporating the invention;

Fig. 2 is an end view of the compressor casing in Fig. 1;

Fig. 3 is a longitudinal sectional view of a somewhat modified compressor inlet casing incorporating the invention;

Fig. 4 is a diagrammatic representation of suitable geometric proportions for the inlet passage;

Fig. 5 is a design data curve illustrating the method of designing the compressor inlet passage;

Fig. 6 is a longitudinal sectional view of a modified form of the invention; and e Fig. 7 is a transverse sectional view on the plane 7-7 in Fig. 6.

Generally stated, the invention is practiced by providing an axial flow compressor with a flaring inlet casing having an annular inlet extending entirely around 360 of the casing, surrounded by a generously proportioned annular plenum chamber disposed around the exterior of the compressor casing and having a radially directed inlet 2 opening, with axial support struts disposed at the inlet to the compressor casing where fluid velocity is low and a comparatively long flow path is provided between the struts and the rotor inlet.

Referring now more particularly to Fig. 1, the invention is illustrated as applied to an axial flow compressor 1, as used, for instance, in a gas turbine powerplant. The compressor has a multi-stage rotor 2, the mechanical details of which are not material to an understanding of the present invention. It may be noted that the rotor has a shaft end portion defining a journal 3, supported in a suitable bearing 4, provided with appropriate shaft seals 5, 6. The bearing housing 4 may, of course, be formed in two halves connected by a horizontal flanged joint, in a purely conventional manner the details of which need not be discussed here. As will be seen in Fig. 1, the bearing housing 4 is supported by a boss or annular flange 7 formed integral with the inner surface of the compressor inlet casing 8.

The support means for the compressor casing may comprise a number of circumferentially spaced pad orboss portions 9, one of which may be provided with a lifting eye 9a as shown in Fig. 1. Other similar pads, either at the horizontal plane through the axis of the rotor, or at other suitable locations around the periphery of the lower half of the casing, may be provided to engage suitable foundation or support pedestal means, one of which is illustrated at 10. With this arrangement, it will be seen that forces imposed by the rotor 2 are transmitted from the bearing 4 directly through the inner wall of the compressor casing 8 to the support pads or bosses 9, thence to the foundation or pedestal 10.

The outer wall of the compressor inlet casing is indicated generally at 11, having at its right-hand end a radially projecting flange portion 11a which may be bolted or otherwise secured to a succeeding compressor casing section 11b. The left-hand end portion of the outer compressor casing wall 11 flares gradually outward with an enlarged bell-mouth portion 110.

It is important to note that the outer stator casing portion 11 is supported from the inner casing wall 8 by a pluralityjof axially extending struts 12, disposed across the maximum diameter portion of the compressor inlet passage, for reasons which will be noted more particularly hereinafter. As will be apparent in Fig. 1, the outer compressor casing 11 may be provided with a plurality of axially extending reinforcing ribs 11d, which extend from the flange 11a axially to the bell-mouth portion 110. The ribs 11d may either join the inlet portion in aligned relation with the ribs 12, or may be spaced circumferentially therebetween. As shown in Figs. 1 and 2, it has been found preferable to use seven of these circumferentially spaced support ribs, one, identified 12c, being located at the bottom centerline of the casing.

The rotor has a plurality of circumferential rows of blades, the initial set being shown at 2a. The fluid flow into this initial rotor blading is directed by a circumferential row of airfoil-shaped inlet guide vanes 13.

It will be apparent from Figs. 1 and 2 that the compressor casing 8, 11 defines an annular inlet extending circumferentially 360 around the rotor. It will be noted that this is not a scroll passage of changing cross-section and shape, but is formed by inner and outer surfaces of revolution, the geometric constitution of which will be seen more particularly hereinafter in connection with Figs. 4 and 5. In other words, the inlet passage between the support struts 12 and the guide vanes 13 is of constant shape and effective flow area, thus helping to insure completely uniform flow approaching the inlet guide vanes 13.

An important part of the invention lies in the fact that the annular compressor casing inlet is fed by a low velocity inlet hood 14, which may be conveniently fabricated of comparatively thin sheet metal Welded together and secured to the compressor casing by a circumferential row of threaded fastenings shown at 11d, 12a, respectively. As may be seen in Figs. 1 and 2, this sheet metal inlet hood 14 comprises a lower half having a semi-cylindrical circumferentially extending wall 140 and flat end walls 14b, 14c disposed in planes transverse to the rotor axis. The upper half of the hood is formed of flat transverse end walls 14d, 14a, and flat side walls 14f (Fig. 1) disposed in planes parallel to a vertical plane through the axis of the rotor. Thus the radial inlet passage is a rectangle bounded by the flat plates 14d, 14e, 14f having a flange 14g to which is bolted the rectangular inlet conduit shown at 14h in Fig. 1.

Thus it will be seen that the invention provides an inlet hood receiving fluid flow in a generally radial direction relative to the axis of the rotor, and providing a generously proportioned annular plenum chamber arranged to supply fluid uniformly around 360 of the compressor inlet casing. It is particularly to be noted in Fig. 1 that this inlet hood is wrapped back over the compressor casing 11, so the use of this generously proportioned plenum chamber produces no overall axial lengthening of the compressor. The cross-section shape of this annular plenum chamber in the lower half of the casing 14a is represented by the dotted lines 11c, 11d, 12h in the lower portion of Fig. 1. Since the outer circumferential casing wall 14a is semi-cylindrical, the cross-section area and shape of this plenum chamber is constant around 180 of the lower half of the casing, from the horizontal flange joint 15.

The flow arrows in Figs. 1 and 2 will indicate generally the fluid path in the inlet hood 14. It will be apparent by comparing Figs. 1 and 2 that the portion of the annular compressor inlet defined between the radial struts 12a, 12b will, generally speaking, be fed by the portion of the inlet duct indicated by the bracketed arrow A. The portion of the annular compressor inlet defined between the struts 12b, 12c will be fed by the portion of the inlet duct indicated by bracketed arrow B in Fig. 2. The air flowing adjacent the end wall 14 will tend to continue circumferentially as indicated by bracketed arrow C to the portion of the compressor inlet between struts 12c, 12d. In contrast, the lowermost arcuate portions of the compressor inlet, between struts 12d, 12e, will be fed principally or entirely from the annular plenum chamber, specifically the portions defined by the bracketed arrows D, E in Fig. l.

The overall effect of this inlet hood arrangement is that the generously proportioned plenum chamber surrounding the compressor casing serves to provide a flow path of greatly increased effective area for feeding those portions of the annular compressor casinginlet with respect to which the fluid experiences the greatest amount of turning in entering the compressor casing, namely, the arc between struts 12d, 12c, although the plenum chamber probably also has some effect in helping to feed the arc between struts 12c, 12d. Thus, proper account is taken of the discovery that, in order to avoid excessive losses in a compressor having an inlet conduit disposed radially, a greatly increased flow path area is required to those portions of the annular compressor inlet at the side of the compressor opposite from the radial direction of approach.

Fig. 3 illustrates a slightly modified form of the compressor casing of Fig. 1. Here the inlet duct is not shown, but instead the view illustrates the cross-section shape and area of the cylindrical lower half of the compressor inlet hood. Since this is equivalent to the cylindrical hood portion 14 in Figs. 1 and 2, it is indicated with like reference numerals. The principal difference in the construction of Fig. 3 over that of Fig. 1 is that the axially extending support strut 121 is of greater width in a radial direction than the strut 12 of Fig. 1, and is moved radially inward towards the axis of the compressor so the leading edge 12g is at a smaller radius from the axis of the compressor; and the trailing edge 12h of the strut is substantially aligned with the cylindrical inner surface of the compressor casing 11. A radially extending rib 6a may be provided as shown in Fig. 3 to transfer stresses from the axial ribs 12f to the bearing support portion of the casing 6b. This combination of the radial ribs 6a, axially extending struts 12], and exterior compressor casing ribs 11c provides a particularly rigid casing construction. The shape and aerodynamic characteristics of the flow path in the compressor inlet casing of Fig. 3 will preferably be as described in connection with Figs. 1 and 2 above, and as illustrated more particularly in connection with Fig. 4 hereinafter. Fig. 3 is not intended to be an accurate representation of the preferred aerodynamic design of the inlet passage, but is presented principally to show the shape of the casing 14, and the modified shape and location of strut 12f. Struts as shown in Figs. 1 and 4 are the preferred embodiment, from the standpoint of aerodynamic performance.

Fig. 4 illustrates in diagrammatic fashion what are believed to be optimum geometric proportions for the shape of the flow path from the annular inlet at struts 12 to the row of stationary guide blades 13. All dimen' sions are stated in terms of a unit R, which is equal to twice the radial height of the inlet guide vanes 13. The physical significance of this quantity R will be seen from the following.

Generally speaking, it can be shown that the aerodynamic performance of an annular passage having a radial dimension is roughly equivalent to that obtained with a hydraulicalof the inlet guide vanes 13 is equal to the radius of the throat of an equivalent long-radius nozzle (defined more particularly hereinafter) of circular cross-section.

The location of the inlet guide vanes 13 relative to the axis 2b of the rotor 2 is given by the radius ratio, defined as the root radius of blades 13, identified r divided by the tip radius of the blades, identified r This radius ratio will vary with axial flow compressors of various design, but it will ordinarily have a value of about .7, and will almost always lie in the range from .5 to about .8 (or .9 at the maximum). As shown in Fig. 4, the root radius r is about 1 /6 R and the tip radius r, is about 1% R, corresponding to a radius ratio of about .70.

The following principal dimensions for the inlet passage and inlet hood will be noted, in terms of the quantity R. v V

The semi-cylindrical lower half of the compressor inlet hood 14a is spaced a distance radially of 1 /3 R from the inlet edge 1211 of the strut 12, which is in turn located at a radial distance R from the outer tips of the guide vanes 13. The overall axial length of the outer compressor casing wall 11 from the bell-mouth inlet portion to the leading edge of the inlet vane 13 is R, and the axial width of the compressor inlet passage is /3 R. The overall axial dimension of the compressor inlet hood 14 from end wall 14b to wall is about 4 R.

Of particular interest is the shape of the interior walls of the compressor inlet passage. This is defined by four ellipsoidal surfaces of revolution, identified E E E E The first ellipsoidal surfaces E and E have crosssections with a major semi-axis equal to R, and a minor semi-axis equal to /3 R, both minor semi-axesbeing substantially in the plane of the inlet edge of the compressor inlet guide vane 13. The third ellipsoidal surface E defines, in a radial plane through the axis of the compressor, an ellipse having its major axis in a radial direction and its minor axis located substantially coincident with the inlet edge of the support strut 12. The major semi-axis measures about 1 /2 R from the inlet edge of strut 12 to the radially innermost end of the ellipse. The minor semi-axis is about equal to R. It will also be noted that the ellipsoidal surfaces E and B are connected by the arcuate portion identified at E, as having a radius with a center so'located that the are E, is tangent to both the ellipsoidal surfaces E E It will be noted by those acquainted with the principles of fluid flow that the ellipsoidal surfaces E E forming the inlet to the guide vanes 13 follow the recommended proportions of the American Society of Mechanical Engineers standard long-radius flow nozzle, the design principles of which are detailed in the ASME Power Test Codes, 1949 Supplement on Instruments and Apparatus, part 5, Measurement of Quantity of Materials, chapter 4, Flow Measurement by Means of Standardized Nozzles and Orifice Plates.

It remains to note the flow area relationships along the mean flow path from the leading edge 12h of strut 12 to the leading edge of guide vane 13. The change in the effective area of this flow path is illustrated graphi cally in Fig. 5. The ordinate represents relative flow path area, measured in a direction normal'to the mean flow path,v and the abscissa is distance along the mean flow path, measured in arcuate degrees from the zero position, at the leading edge 12h of strut 12, to the 90 position, at the leading edge of vane 13. The

location of the 225, 45, and 62.5 positions is illustrated in Fig. 4 by the correspondingly identified lines radiating from a center substantially at the intersection of the leading edge 12h of strut 12, extended to meet the plane of theleading edges of vanes 13. It will be apparent from Fig. 5 that the flow path area decreases smoothly and progressively, from a maximum value at the inlet to the struts 12 to a minimum approaching the guide vanes 13, the importance of which will be noted hereinafter.

The aerodynamic characteristics of the fluid flow in the compressor inlet casing from the strut 12 to the vane 13 may be outlined as follows: It is significant to note that the air-foil section support struts 12 are disposed at the very entrance to the annular compressor inlet, where the eliective flow path area is at its maximum value, meaning that the relative velocity of the fluid past the struts is at a minimum. Thus turbulence and the resulting aerodynamic losses occasioned by any variation in the angle of attack of the fluid approaching the leading edge 12h will have a minimum efi'ect in producing wakes leaving the trailing edge 12i. In this connection, it is also important to note that the extremely generous proportions of the plenum chamber formed by the inlet hood 14 results in a low velocity of approach to the annular compressor inlet, which also reduces the tendency to create strong wakes downstreamfrom struts 13. It is likewise important that thestruts 12 are spaced as far as possible from the guide vanes 13, so that a free flow path of maximum length is provided therebetween, so as to provide time for the flow to become uniform, with the elimination of any small wake eflects which may be caused by the struts 12.

The gradually decreasing flow path area, represented by Fig. 5, means that the fluid is smoothly accelerated from the struts 12 to the guide vanes 13. This accelerating action also has an important tendency to smooth out the flow and eliminate the effect of any eddies or wakes produced by struts 12. The long radius flow nozzle formed by the ellipsoidal surfaces E E provides 'opti' mum flow into the guide vanes 13.

Experience with compressor inlet casings having passages of the shape described reveals that they have very low losses and result in a minimum tendency for the support struts 12 to produce wakes which persist all the way to the vanes 13, which if they occurred would have a serious effect in imposing vibration stimuli on the vanes 13, and on the initial 'row of rotor blades 2a. The smoothly accelerating effect of the inlet passage, terminating in the etficient long radius contracting nozzle immediately adjacent the inlet to vanes 13, produces very uniform flow with a relatively constant angle of attack to the guide vanes 13. Thus the vanes 13 can be made of an air-foil shape, and set at such an angle as to exactly match the uniform fluid flow approaching them.

While in general it is believed advantageous to arrange the support struts 12 in a radial planev through the axis of the rotor, experience with a particular casing may show that the flow can be improved still further by slight adjustments in the angle which the struts 12 make relative to a radial plane. That is, tlieparticular characteristics of the flow in the inlet duct 14h approaching the compressor inlet hood 14 may dictate some small changes in the angle of the struts 12. The proper orientation can then be readily. determinedby simple tests of a model. 1 V

It may also be noted that in the event of extremely non-uniform flow in the inlet duct 14h, it may help to extend the strut 12c radially all the way to the circumferential wall 14a, as shown in Fig. 2 and axially the entire length of the hood from the end wall 14b to the end wall (as shown in Fig. 6). The strut 122, thus extended, becomes a dam, extending entirely across the inlet hood in order to prevent circulation of air in a circumferential direction in the lower part of the hood. The presence of the dam 12a will help to insure that the flow is as represented by the flow arrows in the drawings.

While the number of axially disposed support struts 12 in the compressor casing inlet may be other than shown in the drawings, the number seven is particularly appropriate, as will be seen from the following.

It has been noted that the struts 12a, 12b, 12c and a strut similar to 12c at the lefthand side of the vertical centerline, divide the upper half of the compressor casing inlet into three inlet arcs, two of which are represented by the compressor inlet flow portions identified A, B in Fig. 2. Likewise, the struts 12c, 12d, 12c, and corresponding struts not shown at the left side of the center may be said generally to divide the lower half of the compressor inlet into four arcs, supplied by the portions of the flow identified by arrows D, E, F in Fig. 1. Thus, generally stated, it may be said that the forward portion of the compressor inlet duct 14h represented by bracketed arrow A in Fig. l supplies the three arcs comprising the upper half of the compressor casing inlet, while therearward portion of the inlet duct represented by bracketed arrow D in Fig. 1 supplies the four arcs forming the lower half of the compressor inlet. It will also be noted that the flow represented by. arrows A, B in Fig. 2 approach the respective compressor inlet arcs in a generally radial direction, the flow B having only a comparatively small tangential component relative to the compressor inlet are between struts 12b, 120. On the other hand, the flow represented by arrow C in Fig. 2 has a very substantial tangential component relative to the inlet are between struts 12c, 12d. In contrast, the are between struts 12d, 12c receives little or no flow in a tangential direction, but instead is fed entirely from the plenum chamber, as represented by the flow arrows D, E in Fig. 1. Thus the flow into the inlet are between 12d, He is generally as indicated by the flow arrows F in Figs. 1 and 2. This flow approaches the inlet are by flowing circumferentially around the compressor casing as indicated'by arrows D, E, being required to turn approximately 90" from a circumferential to an axial direction, then turning another 90 from an axial direction into a radial direction, this last turn being represented by arrow F.

This rationale suggests a still further modification of the strut arrangement, as-shown in Figs. '6 and 7.

In Fig. 7, it will be seen that the struts 12a, 12b are precisely as shown in Fig. 2, and the flow represented by arrow A is as in Figs. 1 and 2. On the other hand, the strut 12c of Fig. 2 has been replaced with a radially extended strut 12 which projects radially outward to the casing wall 14]. Thus strut 12) forms a dam forcing all the flow represented by bracketed arrow B in Fig. 7 to enter the inlet are between struts 12b, 12 Thus it may be said that the upper half of the compressor inlet is fed by the forward portion of the inlet duct represented by A in Fig. 1, roughly the middle third of this portion of the duct feeding the are between struts 12a, 12b, and

the respective outer thirds feeding the are between 12b,

12f, and the corresponding arc at the left side of the vertical centerline. Thus, the portion of the flow represented by arrow C in'Fig. 2 (which as noted above has a substantialtangential component relative to the inlet are between struts 12c, 12d) is eliminated, being forced to enter the are between struts 12b, 12f in Fig, 7.

In Fig. 7, the sruts 12g are likewise extended radially to the casing wall 14a. Thus the struts 12f, 1.2g in Fig. 7 define an inlet are in which all the flow is turning 90 from an axial directioninto a dicated by arrow H. The struts 12g, 121 define a similar inlet arc in which all the fiow is turning 90 from an axial to a radial direction, indicated by arrow F. vAs noted previously in connection with Figs. 1 and 2, the lowermost strut 12e of Figs. 6 and 7 is extended radially to meet the casing wall 14a and extends axially all the way from the front wall 14b to the rearmost wall 140, so as to form a complete dam preventing circumferential flow from one half of the casing to the other. On the other hand, the struts 12f, 12g in Fig. 7 do not extend the complete axial length of the casing, but may have an. upstream edge as identified at 12 in Fig. 6, which is an irregular section taken on the broken plane 66 in Fig. 7.

Thus it will be seen that with this modified form of the inlet hood, the flow approaching the three inlet arcs comprising the upper half of the compressor inlet is in an exactly or generally radial direction, approaching the compressor inlet through the forward portion of the inlet duct 14h as represented by while the flow approaching all four of the arcs representing the lower half of the compressor inlet is in a generally radial direction, turning 90 from an axial direction to a radial direction in the process. Thus, the upper half of the compressor inlet has a substantially uniform flow pattern of one type, and the lower half has a substantially uniform flow pattern of a different type. By thus reducing the number of types of flow patterns entering the compressor casing, the problem of achieving the desired degree of uniformity is made easier.

By combining the generously proportioned inlet hood 14, to provide low velocity and uniform angle of approach to the annular compressor inlet, providing the axially extending support struts 12 in the low velocity inlet portion of the compressor casing, together with the long flow path of progressively decreasing effective area from the strut 12 to the guide vane 13, with the terminal portion of this flow path in the form of an extremely efficient long radius contracting nozzle, the invention provides a compressor inlet casing of maximum efficiency, minimum tendency to produce discontinuities in the flow which might result in guide vane or rotor blade failure, while requiring only a very small increase in the radial dimensions of the compressor, and no increase in the overall axial length of the compressor.

radial direction, in-

the flow arrow A, in Fig. l,

Other modifications and substitutions of equivalents will be apparent to those acquainted with axial flow compressor design, and it is of course intended to cover by the appended claims all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an axial flow compressor having a bladed rotor and a generally cylindrical stator casing with an annular row of stationary guide vanes at the inlet to the rotor, an inlet casing assembly comprising radially spaced inner and outer inlet passage walls each forming a surface of revolution about the axis of the compressor and together defining a circumferentially extending radially directed inlet of substantially constant shape and size and at a radius from the axis of the compressor substantially beyond the outer circumference of the stator cas' ing, a plurality of circumferentially spaced struts extending axially across said annular inlet, said struts being disposed in generally radial planes through the axis of the rotor and connected to said inner and outer passage walls at a radius from the compressor axis beyond the tips of the guide vanes, and walls defining a semicylindrical plenum chamber surrounding the inlet end of the axial flow compressor and including a first end wall lying in a plane normal to the axis of the compressor and substantially at the extreme inlet end thereof, the opposite end of the plenum chamber being defined by a wall substantially in a plane normal to the axis of the compressor and at a location spaced axially from said first end wall by a distance on the order of eight times the radial length of the inlet guide vanes, the outer circumferential wall of half of said plenum chamber being generally semi-cylindrical and of a diameter on the order of eight times the radial length of the inlet vanes plus the diameter of the stator casing, the remaining half of the plenum chamber circumference defining a radial inlet passage of rectangular cross-section with generally parallel side walls, said plenum chamber forming a low velocity supply chamber communicating fluid uniformly to the circumferential compressor casing inlet.

2. An axial flow compressor with an inlet casing in accordance with claim 1 in which the surfaces of revolution defining the fiow path from the axial struts to the rotor inlet guide vanes comprise an outer surface the cross-section shape of which is one-quarter of a first ellipse having its major semi-axis parallel to the axis of the compressor and disposed substantially at the same radius from the compressor axis as the leading edge of the axial strut, said first ellipse having a minor semi-axis disposed substantially in the plane of the leading edge of the rotor inlet guide vane, an inner wall having a crosssection shape substantially that of a portion of an ellipse of substantially the same size as said first ellipse but disposed with its major semi-axis parallel to the axis of the rotor and lying between the rotor axis and the inlet guide vanes, and a third surface with a cross-section substantially that of one-quarter of a third ellipse having its major semi-axis in a plane normal to the axis of the compressor and its minor semi-axis substantially at the same radius from the compressor axis as the leading edge of the axial strut.

3. An axial flow compressor with an inlet casing in accordance with claim 1 in which the flow path from the axial struts in the circumferential compressor casing inlet to the compressor inlet guide vanes is defined by ellipsoidal surfaces of revolution shaped and disposed as defined by the surfaces E E E and E in Fig. 4.

4. An axial flow compressor inlet casing in accordance with claim 3 in which the effective cross-section area of said flow path from the axial struts to the inlet guide vanes decreases progressively from a maximum at the struts to a minimum at the guide vanes, whereby smooth acceleration of the fluid along said flow path tends to smooth out the flow and eliminate any wake effects created by the axial struts.

5. An axial flow compressor with an inlet casing in accordance with claim 1 in which the axial struts in the circumferential compressor inlet are seven in number, including a first strut disposed in a central radial plane at the side of the plenum chamber directly opposite the radial inlet passage, said first strut extending from the compressor inlet radially to the cylindrical outer wall of the plenum chamber and axially the entire length of the plenum chamber, whereby the first strut forms a dam preventing circumferential flow in the plenum chamber from one side thereof to the other, second and third struts spaced from opposite sides of the first strut by a distance on the order of one-seventh the circumference of the compressor inlet, fourth and fifth struts disposed at opposite sides of the first strut and spaced from the second and third struts respectively by a distance on the order of one-seventh the circumference of the compressor inlet, and sixth and seventh struts spaced at either side of the central plane extending through the first strut and the axis of the compressor and spaced apart a distance on the order of one-seventh the circumference of the compressor inlet, whereby the sixth and seventh struts divide substantially half of the circumferential compressor inlet into three inlet arcs in direct communication with the rectangular inlet passage, while the first, second, third, fourth, and fifth struts divide substantially the other half of the compressor inlet into four inlet arcs supplied with fluid in a generally axial direction from the portion of said plenum chamber circumferentially surrounding the compressor stator casing and communicating with that portion of the rectangular inlet passage to the rear of the portion of the inlet passage supplying said first three arcs.

6. An axial fiow compressor with an inlet casing in accordance with claim 1 in which the axial struts in the circumferential compressor inlet are seven in number, including a first strut disposed in a central radial plane at the side of the inlet hood directly opposite the radial inlet passage, said first strut extending from the compressor inlet radially to the cylindrical outer wall of the inlet hood and extending axially the entire length of the inlet hood whereby the first strut forms a dam preventing circumferential flow in the inlet hood from one side thereof to the other, second and third struts spaced from opposite sides of the first strut by a distance on the order of oneseventh the circumference of the compressor inlet and extending radially from the compressor inlet to the cylindrical outer wall and extending axially from the compressor inlet end wall a distance substantially on the order of the overall axial length of the circumferential compressor inlet, fourth and fifth struts disposed at opposite sides of the first strut and spaced from the second and third struts respectively by a distance on the order of one-seventh the circumference of the compressor inlet, said fourth and fifth struts likewise extending from the compressor inlet radially outward to the cylindrical wall of the compressor inlet hood and axially a distance on the order of the overall axial length of the circumferential compressor inlet, and sixth and seventh struts spaced at either side of the central plane extending through the first strut and the axis of the compressor and spaced apart a distance on the order of one-seventh the circumference of the compressor inlet, whereby the sixth and seventh struts divide substantially half of the circumferential compressor inlet into three inlet arcs in direct communication with the rectangular inlet passage, while the first, second, third, fourth, and fifth struts divide substantially the lower half of the compressor inlet into four inlet arcs supplied with fluid in a generally axial direction from the portion of the plenum chamber circumferentially surrounding the compressor stator casing and communicating with the portion of the rectangular 10 inlet passage rearward from the portion of the inlet passage supplying said first three inlet arcs.

7. In an axial flow compressor'having a bladed rotor and a generally cylindrical stator casing with an annular row of stationary guide vanes disposed at the inlet to the rotor and having a radial length of an inlet casing assembly comprising radially spaced inner and outer inlet passage walls each forming a surface of revolution about the axis of the compressor and together defining a circumferentially extending radially directed inlet passage of substantially constant shape and size, a plurality of circumferentially spaced struts extending axially across said annular inlet, said struts being disposed in generally radial planes through the axis of the rotor and being connected to the inner and outer passage walls at a radius from the rotor axis beyond said guide vanes, and walls defining a semi-cylindrical plenum chamber surrounding the inlet end of the axial flow compressor and including a first end wall lying in a plane normal to the axis of the rotor and substantially at the extreme inlet end of the compressor casing, the opposite end of the plenum chamber being defined by a wall substantially in a plane normal to the axis of the compressor and connected to the periphery of the compressor stator casing at a location spaced axially from the first end wall a distance on the order of 4 R, the outer circumferential wall of half of the plenum chamber being generally semi-cylindrical and of a diameter on the order of 4 R plus the diameter of the stator casing, the remaining half of the plenum chamber circumference defining a radial inlet passage of rectangular cross-section with generally parallel side walls spaced apart by a distance equal to the diameter of the cylindrical plenum chamber wall, and end walls spaced apart a distance on the order of 4 R, the plenum chamber forming a low velocity supply chamber communicating fluid uniformly throughout 360 to the annular compressor casing inlet.

8. In an axial flow compressor having a bladed rotor and a generally cylindrical stator casing with an annular row of stationary guide vanes at the inlet to the rotor, an inlet casing assembly comprising radially spaced inner and outer inlet passage walls each forming a surface of revolution about the axis of the compressor and together defining a circumferentially extending radially directed inlet of substantially constant shape and size and at a radius from the axis of the compressor substantially beyond the outer circumference of the stator casing, a plurality of circumferentially spaced struts extending axially across said annular inlet and connected to said inner and outer passage walls at a radius from the compressor axis beyond the tips of the guide vanes, said outer inlet passage wall having a cross-section substantially the shape of one-quarter of an ellipse having its major semi-axis parallel to the axis of the compressor and located substantially at the radius from the compressor axis of the leading edge of said axial strut and a minor semi-axis disposed radially and substantially in the plane of the leading edges of the compressor inlet guide vanes, said inner inlet passage wall having a cross-section substantially that of a portion of an ellipse having its major semi-axis parallel to the axis of the rotor and disposed between the rotor axis and the root of the inlet guide vanes and a minor semi-axis disposed radially and substantially in the plane of the leading edges of theinlet guide vanes, the extreme inlet end Wall surface of said inner passage wall having a cross-section shape substantially that of a portion of an ellipse with a minor semiaxis parallel to the rotor axis and substantially at the radius from the compressor axis of the leading edge of said struts and a major axis disposed radially relative to the compressor axis.

9. An axial flow compressor inlet casing in accordance with claim 8, in which the inner and outer inlet passage walls define a flow path having an effective area which decreases smoothly and progressively from the location of the axial struts to the inlet guide vanes, whereby smooth acceleration of the fluid tends to remove wake effects of the axial struts before the fluid reaches the inlet guide vanes.

References Cited in the file of this patent UNITED STATES PATENTS Allen et a1. Mar. 12, 1946 Wosika July 26, 1955 Oechslin Nov. 22, 1955 Fletcher Feb. 12, 1957 

